If you are eating a varied diet (not the same thing every meal and not the same thing the two days in a row) you probably will not need a multi-vitamin supplement. But most nutritionists still advise people to use a good multi-vitamin supplement.
Why? The daily multi-vitamin is an insurance policy. You do need several nutrients every day since we use them up and we cannot make them; such as minerals and several of the B Complex Vitamins. Sometimes we eat the right variety of foods and still do not get enough of specific vitamins, either the food quantity is too large to get all the nutrients we need or the variety of foods does not contain that vitamin or mineral. So the daily multi-vitamin becomes an insurance policy, used just to top off the tank with the necessary nutrients.
What makes a good multi-vitamin? Most of the national brands are good and provide a balanced set of vitamins and mineralss. You do not want a multi-vitamin that provides more than 100% of the RDA (recommended daily amount) for anything. More than 100% is just a waste of good nutrients. More than 100% can be toxic (Vitamins A, D, E and K are toxic along with some minerals). See the Wikipedia article for more information. Also, you do not need a full set of vitamins and minerals every day; your body is better than that; if you miss a nutrient today your body can make up for it tomorrow; the problem becomes missing essential nutrients all the time. So take your multi-vitamin to ensure your health!